1. Field of the Invention
The present invention relates to a register unit and more particularly to a current register unit for storing a current, and a current register circuit, and image display device using the unit.
2. Description of the Related Art
Organic light-emitting diode (hereinafter referred to as OLED) technology has emerged as a popular flat display technology because of its characteristics of lower cost, lower power consumption, self light-emission wider view angle, and faster response time. An OLED is a current-driven component, whose brightness is determined by current there through.
FIG. 1 shows a schematic structure of a conventional OLED display panel. As shown in the drawing, an OLED display panel 1 is formed by interlacing scan lines (represented as S1˜Sn) and data lines (represented as D1˜Dm). Each set of interlacing scan line and data line controls an OLED. For example, a set of interlacing scan lines S1 and data lines D1 control an OLED 100. The anode and cathode of the OLED are respectively connected with data lines (D1˜Dm) and scan lines (S1˜Sn). According to scan signals on the scan lines (S1˜Sn), OLEDs on the same row (i.e. on the same scan line) are all turned on or off to determine whether video signals on the data lines (D1˜Dm) can be input into the corresponding OLEDs.
Additionally, FIG. 1 shows a driving circuit area of the OLED display panel 1. A scan driver circuit 12 outputs scan signals (or scan pulses) to scan lines S1, S2 to Sn in a predetermined sequence. When a scan signal exists on the scan line, OLEDs of display units 10 on the corresponding row are all turned on, and OLEDs of display units 10 on other rows are all turned off. When a scan line is selected, a data driver circuit 11 outputs the corresponding video signal (a current signal) to m display units 10 on the row corresponding to the scan line through data lines D1, D2 to Dm according to image data for display.
Because brightness of the OLED of the display unit 10 is determined by the current through the OLED, a register unit for storing input current signals is a necessary and fundamental component in the data driver circuit 11.
FIG. 2a shows a circuit of a conventional current register unit. It comprises transistors T1˜T4 and a capacitor CS. The gate of the transistor T1 receives the scan signal SS from the scan driver of the OLED display panel, and the drain of T1 receives the current signal from a control IC. The gates of the transistors T2 and T4 receive the scan signal SS as well.
FIG. 2b shows a sampling mode of the conventional current register unit. When the scan signal SS is at a high voltage level, the transistors T1 and T2 are turned on, and the transistor T4 is turned off. Meanwhile, a voltage at B is raised and unstable until a current I1 through the transistor T3 equals the current signal. The capacitor CS stores the stable voltage at B
FIG. 2c shows a reproduction mode of the conventional current register unit. When the scan signal SS is at a low voltage level, the transistors T1 and T2 are turned off, and the transistor T4 is turned on. The voltage difference stored in the capacitor CS acts as a voltage across gate and source (Vgs) of the transistor T3, so that current I2 is generated and flows through the transistor T3, enabling OLEDs on the display panel to emit light. Therefore, a voltage signal in the capacitor CS is critical. If the voltage in the capacitor CS is changed by any noise after the sampling mode is completed, the current register unit will reproduce a current different from the stored current signal during the reproduction mode.
When a state of a transistor is changed by the scan signal, a voltage variation at a gate of the transistor will cause a voltage signal variation at a source or a drain of the transistor under the effect of parasitic capacitance (i.e. voltage coupling effect). For example, when the transistor T2 is controlled by the scan signal SS, the voltage variation at the gate of the transistor T2 will affect the voltage at B. Therefore, the current I2 flowing through the transistor T3 differs from the current signal I1. This would degrade the performance of the OLED.
The conventional method increases the capacitance of the capacitor CS, such that the current register unit needs longer operating time for storing the input current signal. Therefore, the operating speed of the current register unit is limited.